FIG. 1 illustrates an exploded view of a conventional key switch 1, comprising an upper housing 10, a lower housing 11, a hollow cylindrical housing 12 that substantially extends upwards from a center of the upper housing 10, the cylindrical housing 12 being formed at an inner face thereof with four linear guide grooves 13 that are spaced from each other in 90 degrees apart, the guiding grooves 13 being formed with a beveled step face 14 therebetween, the step faces 14 each having a top of a relatively greater thickness and projecting towards an inner side of the hollow cylindrical housing 12. The switch 1 further comprises a cylindrical key 15 being formed at a bottom thereof with four pentagonal protrusions 16 engaging and moving along the linear guide grooves 13, the pentagonal protrusions 16 each having a tip 22 facing downwards and a bottom edge 17 located between the protrusions 16 and extending upwards along the tips 22 to form inverse-V configurations such that the entire bottom edge 17 of the key 15 forms a serrated periphery. The key 15 may engage an engaging unit 18 therein; the engaging unit 18 is formed with a square aperture 19 therein and includes four triangular protrusions 20 at the bottom thereof. The triangular protrusions 20 form an outer diameter that is in-between the inner diameters formed by opposing ends of the step faces 14 in the hollow cylindrical housing 12. The triangular protrusions 20 each has a beveled face 21 that adapts to the tips 22 of the pentagonal protrusions 16 located at the bottom of the key 15.
The engaging unit 18 is further provided therebeneath with a follower rotary body 23 having a square column 24 at an upper portion that adapts to be inserted into the square aperture 19 of the engaging unit 18 such that the follower rotary body 23 may be driven to rotate by the engaging unit 18. The follower rotary body 23 has a bottom being formed by a disc 25, above that a spring 26 is provided engaging around the square column 24. The spring 26 has an upper end that engages a lower edge of the engaging unit 18. The disc 25 is provided with two opposing raised faces 27 at the outer bottom edge of the disc 25, the raised faces 27 each having a thickness that gradually increases while approaching towards a periphery of the disc 25. The disc 25 is further provided with two separate semi-circular supports 28 at the center thereof. The semi-circular supports 28 each have an arcuate inner edge and are provided with a Z-like metallic strip 29 therebetween, as shown in FIG. 1. The lower housing 11 of the conventional switch 1 is formed with a circular recess 2 for receiving the disc 25 located at the bottom of the follower rotary body 23. The recess 2 is formed with an emboss 3 at the center thereof for engaging the two semi-circular supports 28 located at the bottom of the disc 25. The lower housing 11 is further provided with two metallic conductive contacts 4 each having an end 5 that forms a leaf extending towards the circular recess 2 and elevating slightly upwards. The two conductive contacts 4 are each provided with a screw 6 thereon for connecting electrical leads (not shown.) FIGS. 2A to 2C are cross-sectional views of the conventional key switch 1 illustrating the internal structure of the key switch 1 and the switch transition from open to closed circuits. FIG. 2A illustrates the switch 1 in its state of closed circuit. Under the state of closed circuit, the ends 5 of the two contacts 4 are in contact with the raised faces 27 located at the bottom of the disc 25. The follower rotary body 23 made of insulative material subjects the switch 1 to be in the state of closed circuit. A user may push the key 15 to switch the switch I from the state of closed circuit to the state of open circuit. At this time, the pentagonal protrusions 15 located at the bottom of the key 15 still overlay the triangular protrusions 20 located at the bottom of the engaging unit 18 as that shown in FIG. 2A, and engage the linear guide grooves 13 such that both the key 15 and engaging unit 18 are movable along a perpendicular direction. When the engaging unit 18 moves downward to outside of the step faces 14 of the triangular protrusions 20 located at the bottom of the engaging unit 18, the engaging unit 18 is no longer retrained by the linear grooves 13 because the outer diameter of the triangular protrusions 20 located at the bottom of the engaging unit 18 is smaller than the inner diameter of the hollow cylindrical housing 12 beneath the step faces 14, and the engaging unit 18 is thus now rotatable. Because the spring 26 will exert an upward force on the engaging unit 18 after being compressed by the engaging unit 18, the spring 26 will subject the triangular protrusions 20 located at the bottom of the engaging unit 18 to follow the pentagonal protrusions 16 and serrated edges 17 for upward and rotary motions. While viewing from top, the engaging unit 18 rotates a minute angle in a counterclockwise direction subjecting the triangular protrusions 20 to engage the inverse-V serrated edges 17 of the key 15, such as that shown in FIG. 2B, and driving the follower rotary body 23 to rotate simultaneously.
When the user releases the key 15, the thrust of the spring 26 subjects upward movement of the engaging unit 18 such that the triangular protrusions 20 of the engaging unit 18 urge against the lower edges of the beveled step faces 14 and the engaging unit 18 continues to trace along the lower edges of the beveled step faces 14 in upward and counterclockwise rotary motions. The engaging unit 18 further drives simultaneous rotation of the follower rotary body 23 until the triangular protrusions 18 of the engaging unit 18 re-enter the linear grooves 13. At this time, the pentagonal protrusions 16 located at the bottom of the key 15 again overlay the triangular protrusions 20 located at the bottom of the engaging unit 18; the key 15 and the engaging 18 are further pushed upwards by the spring 26 until the pentagonal protrusions 16 of the key 15 urge against a lip 7 of the hollow cylindrical housing 12, as shown in FIG. 2C. At this time, the follower rotary body 23 is exactly 90 degrees away from the upper and lower housings 10, 11 of the switch 1, and opposing sides of the Z-like metallic strip 29 on the bottom of the disc 25 are in contact with the two ends 5 of the conductive contacts 4, respectively, such that the two contacts 4 are electrically connected by means of the ends 5 and the Z-like metallic strip 29 to enable open circuit of the switch 1.
The user may push and release the key 15 again such that the engaging unit 18 and the follower rotary body 23 may rotate for 90 degrees in a similar manner to cause the ends 5 of the two conductive contacts 4 to be in contact with the raised faces 27 located at the bottom of the disc 25, so as to switch the switch I from the state of open circuit to the state of closed circuit, such as that shown in FIG. 2A.
However, such a conventional construction is of a passive switch type, which relies on pushing motions of the user to switch between the open circuit and the closed circuit and thus fails to provide auto switch features. Therefore, when the switch encounters current overload under the state of open circuit, it is liable to cause electrical sparks and result in safety hazards.